The Mayors Against Illegal Guns report calls on Congress to change federal law to define what constitutes a dealer sale of a firearm. Click on the image to read the report.

Loopholes in the licensing of gun dealers are letting “high-volume sellers” flood the U.S. market with tens of thousands of firearms sold through websites or gun shows, without background checks, the Mayors Against Illegal Guns coalition said in a report released Thursday.

The report, released just days ahead of the anniversary of the Newtown school shootings, notes that federal law requires that people “engaged in the business” of firearms sales must have federal licenses – but doesn’t define what that phrase means.

The report, which alleged that at least some of those weapons being sold are ending up in the hands of criminals, called on Congress to close that loophole and to take other steps to strengthen background checks of buyers. Such checks are required for sales by dealers who are federal firearms licensees, but not unlicensed gun sellers. (Read the section of federal law defining a firearms dealer here.)

The coalition, which advocates for additional gun control laws, said that up to 40 percent of the 6.6 million sales, trades or other transfers of guns in 2012 were conducted by private sellers. The report said that a huge increase in online sales has exacerbated the problem.

"The result: A massive online, largely unregulated, secondhand firearms market that threatens the safety of all of us," said New York Mayor Michael Bloomberg, unveiling the report titled “In the Business, Outside the Law.”

The National Rifle Association and the Bellevue-Wash.,-based Second Amendment Foundation did not immediately return calls from NBC News seeking comment about the report.

Bloomberg and the report singled out Armslist.com as a website used by what they called “high-volume sellers” who, they argued, should be required to hold federal firearms licenses. The Gun Control Act excludes private sellers selling weapons from their own collections from having to hold licenses, but it does not define when a private seller crosses the line to become “engaged in the business.” There are no rules about how many guns can be sold in a year, for instance. The mayors’ coalition report also notes that courts have interpreted the law differently as a result.

A telephone request for comment to the Oklahoma law firm listed as the registered agent for Armslist LLC and a request for comment submitted through Armslist.com's online contact page were not immediately returned.

The report said that gun sellers are also circumventing dealer licensing rules at gun shows. It said that because background checks were avoided in these online and gun show sales, weapons were winding up in the hands of people with criminal records who couldn’t otherwise legally own weapons.

The report cited the case of a man arrested in Washington state in 2010 who was accused of selling a .223-caliber semiautomatic rifle at a gun show a week before the weapon was used to kill a Seattle police officer and severely wound another as they sat in their patrol car. He pleaded guilty to charges of selling to people prohibited from owning guns and was sentenced to 18 months in prison.